It was changed well before the ability to use on KE. See my archive.org link above from Feb 2017 --
- Valid on all KLM-marketed, KLM-operated flights from published Main Cabin fares in Y, B, or M class to Business Class.
That was a different change. This was the change to BM fares requiring O to upgrade and only Y getting to upgrade to Z.
The change you mention is where it went from all KL coded being eligible to just YBM. However all still upgraded to Z.